1. Field of the Invention
The present invention relates to a radiographic imaging device that captures a radiographic image expressing radiation that has passed through an imaging subject, a radiographic imaging system, a method of controlling detection sensitivity to radiation irradiation start and a storage medium stored with a program.
2. Description of the Related Art
Recently, radiation detectors such as Flat Panel Detectors (FPDs) are being implemented in which a radiation sensitive layer is disposed on a Thin Film Transistor (TFT) active matrix substrate and with which radiation can be converted directly into digital data. Radiographic imaging devices that employ such radiation detectors to capture radiographic images expressing irradiated radiation are also being implemented. Conversion methods for converting radiation into electric signals used by such radiation detectors include for example indirect conversion methods, in which radiation is first converted into light with a scintillator and then the converted light is converted into charge by a photodiode, or direct conversion methods in which radiation is converted into charge with a semiconductor layer containing for example amorphous selenium. There are various materials that may be used in the semiconductor layer for each method.
In radiographic imaging devices equipped with FPDs, it is necessary to perform synchronization control between the FPD and a radiation source in order to match the start of an accumulation operation, in which the FPD accumulates signal charge, to an irradiation timing of irradiation of radiation from the radiation source. In order to synchronize the timing for the start of radiation irradiation and the timing for the start of the accumulation operation of signal charge by the FPD, a controller such as a console that controls the radiographic imaging device receives an irradiation start signal generated by an irradiation switch connected to the radiation source and supplies this signal to the radiographic imaging device as a synchronization signal. The radiographic imaging device transitions to the accumulation operation and starts imaging on receipt of this synchronization signal.
However, in cases where an imaging system is configured including a radiographic imaging device and a radiation source, sometimes a synchronization control interface installed as standard in the radiographic imaging device or the console thereof (for example cable or connector standards, synchronization signal format) is not compatible with an interface of the radiation source. Due to such issues, radiographic imaging devices are being developed that include an automatic radiation detection function, with radiation irradiation start automatically detected by the radiographic imaging device itself, without the use of a synchronization signal.
For example, Japanese Patent Application Laid-Open (JP-A) No. 2011-185622 discloses a radiographic imaging device provided with: plural radiation detection elements arrayed in a 2D formation in each region of regions partitioned by plural scan lines and plural signal lines; current detection means that detects current flowing in a bias line for applying a bias voltage to the radiation detection elements; control means that detects radiation irradiation start based on a value of the current detected by the current detection means; and memory pre-stored with change profiles of the current detected by the current detection means during reset processing of each of the radiation detection elements. The control means detects radiation irradiation start based on a value ΔV that is the value of the current detected by the current detection means during the reset processing of each of the radiation detection elements reduced by a value corresponding to a value of the current in the change profile.
In radiographic imaging devices with automatic radiation detection functions such as those described above, since the FPD cannot be forewarned of the timing of radiation irradiation, in an irradiation standby state the power is constantly in an ON state and an alert state is maintained until radiation irradiation. There is accordingly significantly increased power consumption in comparison to techniques in which imaging is synchronized to a radiation source. In particular, portable radiographic imaging devices (electronic cassettes) are often driven by a rechargeable battery, with demand to suppress the power consumption and lengthen the operating time for each recharge. However, in radiographic imaging devices, for example, the dose of radiation irradiated onto the FPD differs, for example, according to exposure conditions such as tube current and tube voltage, and such factors as the body thickness of the imaging subject. The radiation irradiation amount irradiated onto the FPD becomes smaller for a larger body thickness of imaging subject than for a smaller body thickness, and hence a higher detection sensitivity to radiation needs to be set. However, if radiation detection sensitivity is set constantly high then, in addition to a large power consumption in the irradiation standby state, there is also concern that false detection of radiation irradiation start due to such influences as noise or vibration might occur.